Such devices for longitudinally guiding a motor vehicle are also known as ACC (adaptive cruise control) systems and typically have a radar sensor as a sensor system which can be used to measure the distances and relative speeds of preceding vehicles. This method makes it possible to follow a vehicle traveling directly ahead, the so-called target object, at a suitable distance or, more precisely, in a suitably selected time interval. In free driving mode when no target object is present, the speed is regulated to a setpoint speed, which in the systems in use today is specified by a desired speed selected by the driver.
In specific conditions, for example when driving through tight curves, it is possible that desired speed may not be adapted to the current situation so that the curve is taken at an excessively high speed. The driver is then forced to intervene in the longitudinal guidance and temporarily deactivate the ACC system.
In German Patent Application Nos. DE 198 21 803 A1 and DE 199 31 161 A1, longitudinal guidance systems are described having a connection to a navigation system which is also present in the vehicle so that the route information supplied by the navigation system, in particular the information readable from a digital map concerning the curvature of the section of roadway directly ahead, may be included in the speed regulation. However, there is the problem that at intersections or forks in the road in the complex road network it is not always clear a priori which route the host vehicle will take. The route information provided by the navigation system may be used in longitudinal guiding only if the guidance function of the navigation system is active, so it may be assumed that the host vehicle will follow the route calculated by the guidance function. However, most trips with a motor vehicle take place in known territory, so that the guidance system is not necessary per se and would only have to be activated for the purpose of longitudinal guidance which involves, however, a relatively cumbersome destination input. Moreover, even when the guidance system is activated, it is relatively frequently the case that the driver deviates from the calculated route because the calculations executed by the navigation system for finding the quickest route to the destination are based on certain assumptions about the speeds possible on highways, country roads, or inner city streets and these assumptions frequently contradict the driver's experience.